Medical implants are used for innumerable medical purposes, including the reinforcement of recently re-enlarged lumens, the replacement of ruptured vessels, and the treatment of disease such as vascular disease by local pharmacotherapy, i.e., delivering therapeutic drug doses to target tissues while minimizing systemic side effects. Such localized delivery of therapeutic agents has been proposed or achieved using medical implants which both support a lumen within a patient's body and place appropriate coatings containing absorbable therapeutic agents at the implant location. Examples of such medical devices include catheters, guide wires, balloons, filters (e.g., vena cava filters), stents, stent grafts, vascular grafts, intraluminal paving systems, implants and other devices used in connection with drug-loaded polymer coatings. Such medical devices are implanted or otherwise utilized in body lumina and organs such as the coronary vasculature, esophagus, trachea, colon, biliary tract, urinary tract, prostate, brain, and the like.
The delivery of expandable stents is a specific example of a medical procedure that may involve the deployment of coated implants. Expandable stents are tube-like medical devices, typically made from stainless steel, Tantalum, Platinum or Nitinol alloys, designed to be placed within the inner walls of a lumen within the body of a patient. These stents are typically maneuvered to a desired location within a lumen of the patient's body and then expanded to provide internal support for the lumen. The stents may be self-expanding or, alternatively, may require external forces to expand them, such as by inflating a balloon attached to the distal end of the stent delivery catheter.
The mechanical process of applying a coating onto a stent or other medical device may be accomplished in a variety of ways, including, for example, spraying the coating substance onto the device, so-called spin-dipping, i.e., dipping a spinning device into a coating solution to achieve the desired coating, and electrostatic fluid deposition, i.e., applying an electrical potential difference between a coating fluid and a target to cause the coating fluid to be discharged from the dispensing point and drawn toward the target.
Common to these processes is the need to apply the coating in a manner to ensure that an intact, robust coating of the desired thickness is formed on the stent. Electrostatic coating has been employed to obtain coated medical devices, particularly in applications where the coating fluid viscosity is very low, for example, in the vicinity of one centipoise. For example, in U.S. patent application Ser. No. 10/409,590, filed Apr. 9, 2003, the disclosure of which is hereby incorporated in its entirety by reference, a coating application apparatus and method is described in which a target, such as a stent, is held by a target holder at a first electrical potential. A second potential is applied to an electrode in contact with the coating fluid within a coating fluid spray dispenser to impart a charge to the coating fluid. The charged coating fluid is then accelerated by electrostatic attraction from the spray dispenser toward the target device.
The foregoing approach to electrostatic coating application provides highly uniform coating application, along with other benefits, such as precision control of coating deposition rates and highly efficient production when incorporated into automated device handling systems. However, to maximize efficient utilization of the coating material with this approach, sufficient electrostatic attraction of the coating fluid particles to the target should be provided in order to obtain a high rate of coating deposition, and thus minimize coating waste (i.e., coating that fails to adhere to the target). Obtaining sufficient electrostatic attraction between the target and the coating fluid spray should consist of both (i) good conductivity between the target holder and the target to ensure the first potential applied to the target holder is fully transferred to the target, and (ii) ensuring the coating fluid picks up enough charge as it passes through the sprayer nozzle such that the fluid particles that emerge from the sprayer are sufficiently charged to be attracted to the target.
Empirical experience has shown that the target holder-to-target conductivity can vary significantly on an individual target-to-target basis. Such variability could be detrimental to obtaining consistent coating distribution and thickness on the target. Experimentation with the attachment of high-conductivity materials to the target, such as gold or gold-plated electrodes, to enhance holder-to-target conductivity has not completely eliminated the variability in conductivity. As a result of the experimentation, however, it was discovered that oxide formed on the surfaces of a metal target is a principal source of the inconsistent holder-to-target conductivity, and that elimination of the oxidation at the holder-to-target contact points ensures the target is held at the same potential as its holder to better attract the charged coating fluid spray.
With regard to ensuring a sufficient charge is imparted to the coating fluid, some electrostatic nozzles typically are constructed with a non-conductive housing containing an internal electrode, and the coating fluid is charged by applying the second electrical potential voltage to the internal electrode. The internal electrode arrangement is disadvantageous, however, as it limits the amount of charge than may be efficiently transferred to the coating fluid spray. Moreover, an internal electrode arrangement increases the complexity of the internal arrangements of the nozzle, while the amount of space available for the internal electrode is limited by other nozzle internal parts. There also must be provided an effective electrode-to-dispenser nozzle seal to prevent leakage of the coating fluid from the electrode/nozzle interface. Other disadvantages of internal electrode-type nozzles are increased dispenser manufacturing costs, and increased difficulty in properly cleaning the electrode and the other parts within the dispenser. Further, as a consequence of the internal electrode dispensing nozzle's internal geometry limiting electrode surface area, the amount of charge transfer from the internal electrode to the coating fluid is also limited. This in turn lowers the coating fluid's ionization, which decreases its attraction to the target. Combined with decreased electrical potential at the target due to varying holder-to-target conductivity, the coating fluid's attraction to the target is lower than desired, which decreases the coating deposition rate on the target because a greater fraction of the coating spray passes by or through the target without depositing thereon. The result is a lower overall coating utilization rate, and undesired waste of coating fluid.